Probing for New Physics in Polarized $\Lambda_b$ decays at the Z

Abstract

Polarized $\Lambda_b \to \Lambda \gamma$ decays at the Z pole are shown to be well suited for probing a large variety of New Physics effects. A new observable is proposed, the angular asymmetry between the $\Lambda_b$ spin and photon momentum, which is sensitive to the relative strengths of the opposite chirality and Standard Model chirality $b \to s \gamma$ dipole operators. Combination with the $\Lambda $ decay polarization asymmetry and comparison with the $\Lambda_b$ polarization extracted from semileptonic decays allows important tests of the $V-A$ structure of the Standard Model. Modifications of the rates and angular asymmetries which arise at next-to-leading order are discussed. Measurements for $\Lambda_b \to \Lambda \gamma$ and the CP conjugate mode, with branching ratios of a few times $10^{-5}$, are shown to be sensitive to non-standard sources of CP violation in the $\Lambda_b \to \Lambda \gamma$ matrix element. Form factor relations for heavy-to-light baryon decays are derived in the large energy limit, which are of general interest.